1. Field of the Invention
The present invention relates to a projection lens for magnifying and projecting display information or the like output from a light valve, such as a liquid crystal display device and a DMD (digital micromirror device). Further, the present invention relates to a projection-type display apparatus using the projection lens.
2. Description of the Related Art
In recent years, projection-type display apparatuses using light valves, such as a transmissive or reflective liquid crystal display device and a DMD display device, became widely used. Especially, projection-type display apparatuses using three light valves corresponding to illumination light of RGB primary colors are widely used. In the projection-type display apparatuses using three light valves, rays of illumination light of RGB primary colors are modulated by respective light valves, and combined by a prism or the like. Further, an image is projected onto a screen through a projection lens.
As described above, a prism or the like for combining colors is arranged in the projection-type display apparatus, in which rays of light modulated by three light valves are combined by a color combination optical system. Therefore, a projection lens mounted on the projection-type display apparatus needs to have a long back focus. The long back focus is necessary also to prevent a thermal problem in the apparatus.
Further, in such a projection lens, projection of a large image with a short projection distance is requested. Further, since reduction in the thickness of an apparatus is strongly requested especially in a rear-type projection apparatus, a wide-angle projection lens is requested.
To satisfy such requests, the inventor of the present invention has disclosed a projection lens that has a wide angle of more than or equal to approximately 105 degrees and excellent projection performance, while maintaining a long back focus (please refer to U.S. Pat. No. 7,529,033 (Patent Document 1)).
However, the number of lenses in the projection lens disclosed in Patent Document 1 is in the range of 13 to 16. Therefore, in Patent Document 1, the size and the cost of the projection lens are not reduced in an excellent manner.
Further, reduction in the diameter of a magnification-side lens in the projection lens is requested to reduce the size of the projection lens.
Especially, when a reflective liquid crystal display device or the like is used, a long back focus is requested. However, in the projection lens disclosed in Patent Document 1, the back focus is not always appropriate.